Surface enhanced membrane and process and apparatus for producing same

ABSTRACT

A membrane having a refined surface as well as to a process and an apparatus for producing such a membrane, and which is useful, in particular, for rapid diagnostic tests for identifying specific analytes in liquid media.

BACKGROUND OF THE INVENTION

[0001] The present invention relates both to a membrane composed ofcellulose which has an improved surface, and to a process and anapparatus serving the manufacture of such membrane, the latter havingapplication in particular to rapid diagnostic tests that are used toidentify specific analytes present in fluid media.

[0002] Membranes composed of cellulose are frequently employed in themanufacture of dry reagents, and, more particularly, of test strips thatare used in analysis and in medical diagnostics (diagnostic membranes).U.S. Pat. No. 5, 628,960 discloses both such a symmetrical membranecomprising a polymer blend of cellulose nitrate and cellulose acetatewhich, containing micropores, is supported on a polyester carrier film,and a method for the production thereof. In the prior art rapiddiagnostic tests employed to identify specific analytes, for example inhuman, animal and vegetable organisms, in food and in the environment,the requirement to bind proteins or protein-like substances used asanalytes and the requirement to have large pore sizes, necessitates theuse of membranes containing primarily cellulose nitrate as the dryreaction phase, since only this type of membrane can satisfy both of theaforementioned requirements. Such membranes are chiefly produced fromcritical casting solutions in a so-called evaporation process in whichthe underlying formation mechanism is based on a phase inversion.

[0003] This process permits the manufacture of microfiltration membranescomprising primarily cellulose nitrate and having pore widths rangingbetween approx. 0.01 and at least 12 μm. One disadvantage attending thisprocess is that, beyond a pore width of approx. 0.45 μm, and, moreparticularly with respect to the membranes which being normally used indiagnostic applications, exhibit pore widths>0.45 μm, a distincttendency to form so-called filter dust has been observed, resulting fromthe unwanted fractionated precipitation of the polymers during the phaseinversion process.

[0004] The filter dust fraction is formed from the non-membrane-forming,low molecular portion of the commercially available feedstocks usedwhich precipitate out chiefly in the form of a more or less looselypacked layer of dust or irregularly formed deposits on the surface ofthe membrane and, more noticeably, on the upper side of the heretoforeprepared integrated membrane layer.

[0005] Most of the cellulose nitrate membranes that are employed fordiagnostic purposes are in a film-supported format. Currently, suchmembranes are, for economic reasons, no longer produced by laminatingthe cellulose nitrate membrane onto an adhesive-coated film, but arerather produced by pouring membrane casting solutions directly onto afilm as disclosed in Beer et al., U.S. Pat. No. 5,628,960.

[0006] It will, of course, be appreciated that the appearance of filterdust in such an arrangement is particularly problematic, since duringthe manufacture of such membranes, the casting solution can be appliedonly to the critical upper side of the membrane upon which filter dusthas accumulated. Such deposits for a variety of reasons pose a majorproblem with respect to application in quick diagnostic tests. Due todifferences in structure existing between the membrane layer and thefilter dust deposit, the test liquids that are poured onto the membranewill migrate at different velocities on top or bottom, the resultwhereof being the formation of two separate traveling fronts over abroad, diffuse zone. The filter dust deposit moreover distorts the shapeof the test lines and weakens the intensity of the resulting coloursignals. These problems raise doubt as to the crucial precision andinformation value of the tests with respect to image sharpness,sensitivity, reproducibility and clarity.

[0007] A prior art process for eliminating the aforementioned problemconsists of reprecipitating commercially available feedstocks in orderto remove the low molecular components prior to using such feedstocks inthe manufacture of the membrane. See Perl, German Patent No. DE 37 08946 C1 and Beer et al., U.S. Pat. No. 5,628,960 (=DE 44 38 381). Thedisadvantages attending this multi-phase process are its complexity, itscost and its inefficiency.

SUMMARY OF THE INVENTION

[0008] It is therefore an object of the present invention to provide animproved process for refining the surface of membranes composed ofcellulose.

[0009] Another object of the invention is to provide a process forrefining the surface of a cellulose membrane which to effectivelyproduce membranes which have surfaces that do not exhibit harmfulimpurities such as, for example, filter dust.

[0010] A further object of the invention is to provide a process forproducing cellulosic membranes which can be employed as diagnosticmembranes.

[0011] It is also an object of the invention to provide an improvedapparatus for producing a cellulosic membrane with a refined or enhancedsurface.

[0012] These and other objects are achieved in accordance with thepresent invention by providing a process for producing a cellulosemembrane having a refined surface, comprising the steps of preparing afeedstock membrane from a cellulose membrane casting solution by phaseinversion in an evaporation process, and prior to drying the resultingfeedstock membrane, removing impurities from at least one side of saidfeedstock membrane from which the solvent is evaporated in theevaporation process of the feedstock preparation step.

[0013] In accordance with a further aspect of the invention, the objectsare achieved by providing a surface-refined membrane composed ofcellulose, produced by a process comprising the steps of preparing afeedstock membrane from a membrane casting solution by phase inversionin an evaporation process, and prior to drying the resulting feedstockmembrane, removing impurities from at least one side of the feedstockmembrane from which solvent has been evaporated in the evaporationprocess of the membrane preparation step.

[0014] In yet other aspects, the objects of the invention are achievedby providing an apparatus for refining a surface of a membrane composedof cellulose produced by phase inversion in an evaporation process, theapparatus comprising at least one membrane drawing machine for producinga feedstock membrane by phase inversion in an evaporation process; atleast one membrane cleansing device for bringing at least one side ofthe feedstock membrane from which impurities are to be removed, intocontact with at least one cleansing agent or cleansing device, and atleast one membrane dryer; in which the membrane cleansing device isarranged between the membrane drawing machine and the membrane dryer.

[0015] The invention also includes a method of using strips of membraneaccording to the invention in diagnostic testing of analytes.

[0016] A first embodiment of the present invention relates in particularto a process which, serving the manufacture of a surface-refinedmembrane composed of cellulose, comprises the following steps:

[0017] (a) preparation of a feedstock membrane (crude membrane) from amembrane casting solution using phase inversion in an evaporationprocess, and

[0018] (b) prior to drying the resulting membrane, removal of impuritiesfrom at least that side of the feedstock membrane from which the solventhas been evaporated in the evaporation step mentioned in (a).

[0019] Preparation of the feedstock membrane composed of, for example,cellulose nitrate and/or cellulose acetate described in step (a) of theaforementioned is effected by means of a prior art evaporation process.

[0020] It is proposed that the impurities or deposits such as filterdust referred to in (b) be removed from that side of the feedstockmembrane which, during the evaporation procedure mentioned in step (a)in which solvent components were evaporated, was exposed to theatmosphere. In the case of membranes which, in step (a) are preparedwith a film-supported underside, this is preferably the upper side ofthe membrane.

[0021] It is preferred that step (b) of the proposed process comprisesbringing that side of the membrane, from which impurities are to beremoved, into contact with one or more cleansing agents and/or one ormore cleansing devices. Impurities such as, for example, filter dust,are removed by means of, for example, the upper side of the membranecoming into contact with the cleansing agents/devices, without damagebeing caused to the membrane. At the same time, the upper sides of themembrane are homogenized.

[0022] In order to enhance the effectiveness of the proposed process,one preferred embodiment of the aforementioned process comprises thatthe impurities be removed after the membrane has been cast, but beforeall of the solvent or solvent mixture used to prepare the membrane, havebeen evaporated, since this arrangement affords improved access to thefine structure of the membrane for cleansing purposes. It isparticularly advantageous if impurity removal be carried out at ajuncture, at which the membrane still exhibits between 5 and 20 per centby weight and, more particularly, 10 to 15 per cent by weight, of theoriginal quantity of the solvent or solvent mixture (volatile componentsof the membrane casting solutions employed).

[0023] It is proposed that, for example, cleansing fluids, such asliquids or gases, be considered for use as cleansing agents. Liquidcleansing agents can, e.g. be applied in the form of washing fluids, bymeans whereof the impurities can be removed in a washing step lasting apredetermined amount of time, e.g. wherein the feedstock membrane passesat a preset speed through a bath filled with washing liquid. Anotherembodiment, which permits removal of impurities from the feedstockmembrane, comprises that the cleansing liquid be applied, for example,to the upper side of the membrane in a pressurized stream of liquid. Inthis configuration, it will, of course be appreciated that the streampressure can be regulated so as to permit the effective removal ofimpurities without exerting high mechanical loads upon the membrane whenthe latter is being sprayed with liquid.

[0024] Analogously, the impurities can be removed with the aid of astream of pressurized gas, whereby filter dust, for example, is blownaway. The gas preferred for this embodiment is air.

[0025] It is proposed that preferred cleansing agents contain waterand/or an alcohol, which features preferably from 1 to 6 carbon atoms.Examples of suitable alcohols include methanol, ethanol, propanol, and,more particularly, isopropanol, butanol, pentanol and hexanol.Especially preferred cleansing agents are water and mixtures of waterand the aforementioned alcohols, and, more particularly, such mixtures,in which the ratio of the volume of water to alcohol is 1:1 to 20:1. 5:1to 15:1 is particularly preferred, with 10:1 being most preferred.

[0026] A further advantageous configuration of the proposed processcomprises that the membrane be brought into contact with one or morewipers and/or one or more suction devices serving as (mechanical)cleansing devices.

[0027] One advantage attending the proposed process is its flexibleconfiguration, which allows its adaptation to the individualrequirements of a given membrane, insofar as single, or a plurality ofthe aforementioned cleansing agents or apparatuses or any combinationthereof, can be employed during the removal of the impurities.

[0028] Thus, having application to this object are mechanical wipershaving, for example, wipers or brushes, washing apparatuses havingwashing liquids, in which the membranes are dipped and moved, suctionapparatuses whereby air or a rinsing liquid is streamed over the uppersides of the membrane and, together with the filter dust suctioned off,blowing apparatuses serving to produce a stream of gas or devicesproducing a stream of liquid in order to remove filter dust from theupper sides of the membrane, spraying devices serving to spray washingliquids onto the upper sides of the membrane, and other similar devicesas well as combinations thereof.

[0029] In one preferred embodiment of the invention, the membranes passat step (b)

[0030] (i) through a first and a second washing device so as to bebrought into contact with one or more cleansing agents, and

[0031] (ii) through one or more wipers.

[0032] This embodiment is advantageously configured so as to permit themembrane, immediately upon exiting the membrane drawing machine, to passthrough a first washing device, and wipers, which can be integrated withthe first washing device, and then through a second washing device.

[0033] Membranes composed of cellulose which are used, for example, inmedical diagnostics, are, as a rule, treated with additives or reagentsin order to impart to such membranes certain characteristics requiredfor their use as diagnostic tools. Since the addition of additives tothe membrane casting solution can adversely influence membraneformation, a further preferred embodiment of the invention comprisesimpregnating the membrane with at least one additive.

[0034] The membrane is preferably brought into contact with at least oneadditive or reagent while the membrane is passing through at least one,and preferably the final washing device, wherein an additive or reagentis added to the washing agent. This arrangement is particularlyadvantageous, since it has been found that the additive is at leastpartially removed from both the inner and outer surfaces of the membraneby means of the rinsing agent that is contained in the first washingdevice.

[0035] In this arrangement, membrane thickness, the type andconcentration of the additives as well as the period of time spent bythe membranes in the last washing device, i.e in the impregnation baths,are adjusted relative to one another in order to permit a homogenousdistribution of the additives throughout the membrane layer and atsufficient operational speed for later employment, e.g. as a diagnosticmembrane.

[0036] It is understood that the inner surface refers to the surfaceinside the membrane pores. Additives refers, for example, to substancesserving to alter both the contact angle and the adsorptivecharacteristics of the membrane surface, such as, e.g. wetting agents,other hydrophilic, or, rather, hydrophobic agents, such ashygroscopic-regulating or pH value regulating agents (buffersubstances). The contact angle or wetting angle is that angle formedbetween a drop of liquid (mostly water) and the outer surface of a flatmembrane.

[0037] The aforementioned process permits, more particularly, thepreparation of membranes having improved surface characteristics and apore width from 0.01 to 12 μm. Especially preferred membranes exhibit apore width of greater than 0.45 μm. Prior art systems have been unableto efficiently produce impurity-free membranes possessing pores of thiswidth.

[0038] A further embodiment of the present invention permits, with theaid of the aforementioned process, manufacture of a membrane composed ofcellulose having a refined surface. The resulting membrane is suitable,for example, for use in diagnostics, and, more particularly, indiagnostic test strips.

[0039] A further object of the present invention relates to an apparatusserving to enhance or refine the surface of a membrane composed ofcellulose which has been produced by means of phase inversion as part ofan evaporation process, comprising

[0040] at least one membrane drawing machine that is designed to producea feedstock membrane by means of phase inversion in an evaporationprocess,

[0041] at least one membrane washing device, which, for the purpose ofbringing at least that side of the feedstock membrane from whichimpurities are to be removed into contact with one or more cleansingagents and/or one or more cleansing devices, and

[0042] at least one membrane dryer,

[0043] wherein the membrane cleansing device is arranged between themembrane drawing machine and the membrane dryer.

[0044] A preferred proposed apparatus for carrying out the process forrefining or enhancing the surface of membranes composed of cellulose,such membranes being produced from membrane casting solutions in anevaporation process by means of phase inversion, comprises a membranecleansing device having one or more mechanical wipers and/or suctiondevices and one or more washing devices. As described above, themembrane cleansing device is arranged between a membrane drawing machineserving the implementation of the evaporation process, and a membrane(end) dryer. This configuration permits the upper sides of themembranes, which upper sides are exposed to the atmosphere when thesolvent components are evaporated, to be guided past and/or through themechanical wipers and washing devices, whereby impurities, for example,filter dust, are removed from the membranes when the upper sides of themembranes come into contact with the wipers and the wash liquids of thewashing devices.

[0045] In one preferred embodiment, there are a first washing device,and at least one wiper, as well as a second washing device. The firstwashing device, which is preferably arranged directly downstream of themembrane drawing machine, comprises both at least one wash basin orreservoir containing washing liquid and guide rollers serving to advancethe membranes and being arranged in such a manner that the membranes,when passing through the washing device, are transferred into and out ofthe washing device. The second washing device, which comprises at leastone wash basin or reservoir containing washing liquid and guide rollersserving to transfer the membranes, is preferably arranged directlyupstream of the membrane dryer. Suitable washing liquids which can beused include, for example, water and aqueous solutions or water/alcoholmixtures.

[0046] In a further preferred embodiment of the invention, in order tobring the membrane simultaneously into contact with an additive presentin the washing liquids, which can for example, be water or awater/alcohol mixture that is present inside the wash basin that isarranged upstream of the membrane dryer, at least one additive, e.g. awetting agent is added thereto.

[0047] The mechanical wipers, or, rather, the suction devices, extendpreferably at least over the integral width of the membrane. Oneadvantageous configuration of a suction device comprises a device which,being fitted with a suction slit and developing underpressure, isarranged above the upper side of the membrane.

[0048] In accordance with a further preferred embodiment, the wipers,which are provided with resilient wiper elements, are arranged so as topermit the wiper elements to sweep across the upper side of themembrane, whenever membrane and wipers are moved relative to the wiperelements, thus wiping away the impurities.

[0049] It is preferred that the wiper elements be disposed upon theouter peripheral surface of a rotating wiping roller that rotatespreferably in the direction in which the membrane is drawn, in whichcase the rotation speed is greater than the speed at which the membraneis drawn. In order to prevent the membrane from sustaining mechanicaldamage, a support roller can be used in conjunction with the wiperroller in order to support the membrane. In such an arrangement, themembrane is guided through a nip between the wiper roller and thesupport roller. The latter rotates in the direction of draw at a speedcorresponding to that of the draw. By drawing speed is meant the speedat which the membrane moves through the drawing machine.

[0050] In another embodiment of the apparatus of the invention, themembrane is guided directly over the rotating or immobile wiping device.Particularly suited to this role are wipers, with respect to which theresilient wiper elements comprise rubber wipers and/or brushes. In afurther embodiment of the proposed device, the wipers are integratedwith the first washing device. In the event the enhanced surfacemembranes to be processed are diagnostic membranes, which are applied toa carrier film, the proposed device can feature only one wiper, which isarranged preferably above the washing liquid of the first washingdevice. In another preferred embodiment, the wipers are fitted withnozzles, through which washing liquid is sprayed onto the wiperelements, for the purpose of removing any impurities that may besticking to the wiper elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The invention will be described in further detail hereinafterwith reference to illustrative preferred embodiments shown in theaccompanying drawings, in which:

[0052]FIG. 1 shows a scanning electron microscopic (SEM) photograph(enlarged 400×) of the upper side of a film-supported membrane withoutsurface enhancement which, having been produced by direct coating,exhibits a deposit of filter dust;

[0053]FIG. 2 shows a SEM photograph (enlarged 400×) of the upper side ofa film-supported membrane according to the present invention, and

[0054]FIG. 3 shows a schematic representation of a specific embodimentof an apparatus according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0055] In FIG. 3, the proposed device comprises a membrane cleansingdevice (1) having a first rinsing device (2), a rotating brush servingas a mechanical wiper (3) as well as a second rinsing device (4).Rinsing devices (2) and (4) are fitted with guide rollers (5) serving toguide membrane (6). Membrane cleansing device (1) is arranged between amembrane drawing machine (7) serving the implementation of theevaporation process as well as a membrane dryer (8). First rinsingdevice (2) is arranged immediately downstream of membrane drawingmachine (7) and comprises a wash basin (9) comprising an outlet forwashing liquid (10). Spray nozzles (11,12) are provided for the purposeof spraying fresh washing liquid onto both wiper (3) and the membraneissuing from first rinsing device (2). Second rinsing device (4)comprises two wash basins (13,14) which, being separated by means of apartition wall (15), serve to prevent impurities from being carried intowash basin (14). The washing liquid, to which additives have been added,flows through an inlet (16) into wash basin (14) and drains out of washbasin (13) through outlet (17).

[0056] In order to protect membrane (6) against mechanical damage, asupport roller (18), arranged in addition to wiper roller (3), supportsthe membrane on its film-supported side (19). In this arrangement,membrane (6) is guided between wiper roller (3) and support roller (18).

[0057] In order to realize the proposed process, the film-supportedmembrane (6) is transferred at drawing speed, immediately upon issuingfrom membrane drawing machine (7), over guide rollers (5) and supportroller (18), through the washing liquid of first rinsing device (2). Inthis configuration, the upper side of membrane (6) is brushed by meansof rotating wiping roller (3) configured as a brush, in the presence ofwashing liquid (11) (preferably water or a water/alcohol mixture)whereby most of the impurities e.g. filter dust, are loosened andremoved. Prior to issuing from first rinsing device (2), the upper sideof membrane (6), or, rather, the under side of film (19) is washed ofany impurities that may remain thereupon, by means of washing liquidissuing from nozzles (12). Membrane (6) is, when passing through secondrinsing device (4), into which a washing liquid, to which additives havepreferably been added, enters via inlet (16), impregnated with suchadditives, and thereafter dried in dryer (8).

EXAMPLES Manufacturing Example 1

[0058] A prior art membrane casting solution comprising a polymer blendof commercially available cellulose nitrate and cellulose acetate is, ina membrane drawing machine, applied to a carrier film comprisingpolyethylene terephthalate having a layer thickness of 100 μm. Thenow-coated film is, during its transit through the drawing machine up tothe exit thereof, subjected to evaporation for the purpose of removingthe greater part of the components of the solvent mixture, whereby thecarrier-supported membrane results from phase inversion. The membrane,having being dried in the end dryer (three-roller frame) exhibits bothpores approx. 10 μm wide and filter dust deposits on its upper side(FIG. 1).

Operation Example 1

[0059] The membrane resulting from aforementioned Manufacturing Example1, is transferred, when still damp, from the exit of the membranedrawing machine into the proposed membrane cleansing device that isschematically represented in FIG. 3. Water is used as rinsing liquid.Employed as the mechanical wiper is a roller shaped brush, whosecircumference speed corresponds to approx. 10 times the speed at whichthe membrane is drawn. The membrane is impregnated in the second rinsingdevice with a 0.01 to 1.5% solution of an anionic wetting agent. Themembrane is next dried as described in Manufacturing Example 1. Theresulting membrane, which has pores approx. 10 μm wide, is, however,free of filter dust and other impurities (FIG. 2) and exhibits, whentested with a test fluid, (a solution of phenol red) a migration time ofon average 90 sec./40 mm with an unbroken, straight front of advance andproduces, when applied, a precise, intensely-coloured test line. Themigration speed was measured in the vertical suction direction inaccordance with DIN 53106.

Comparison Example 1

[0060] By way of comparison, the membrane resulting from ManufacturingExample 1, which has been conventionally manufactured and undergone nofurther processing, both exhibits a surface covered with filter dust andproduces, upon migration with test fluid (phenol red solution) a doubleadvancement front with different migration times on both upper or lowersides of the membrane and, when applied, exhibits a diffuse, weaklydelineated test line.

[0061] The foregoing description and examples have been set forth merelyto illustrate the invention and are not intended to be limiting. Sincemodifications of the described embodiments incorporating the spirit andsubstance of the invention may occur to persons skilled in the art, theinvention should be construed broadly to include all variations fallingwithin the scope of the appended claims and equivalents thereof.

What is claimed is:
 1. A process for producing a cellulose membranehaving a refined surface, comprising the steps of: (a) preparing afeedstock membrane from a cellulose membrane casting solution by phaseinversion in an evaporation process, and (b) prior to drying theresulting feedstock membrane, removing impurities from at least one sideof said feedstock membrane from which the solvent is evaporated in theevaporation process of step (a).
 2. A process according to claim 1,wherein the removal of impurities is continued as long as unevaporatedsolvent remains in the membrane.
 3. A process according to claim 1,wherein step (b) comprises bringing the side of the membrane from whichimpurities are to be removed, into contact with at least one cleansingagent or cleansing device.
 4. A process according to claim 3, whereinthe impurities are removed by a cleansing fluid.
 5. A process accordingto claim 4, wherein the cleansing fluid is a gas.
 6. A process accordingto claim 5, wherein the impurities are removed by a pressurized gasstream.
 7. A process according to claim 5, wherein the gas is air.
 8. Aprocess according to claim 4, wherein the cleansing fluid is a liquid.9. A process according to claim 8, wherein the impurities are removed bya pressurized stream of liquid.
 10. A process according to claim 3,wherein the impurities are removed by a cleansing device selected fromthe group consisting of wipers and suction devices.
 11. A processaccording to claim 1, wherein, in step (b), the membrane passes through:(i) a first rinsing device; (ii) a second rinsing device; and (iii) atleast one wiper.
 12. A process according to claim 11, wherein themembrane in the second rinsing device is simultaneously contacted withat least one additive.
 13. A process according to claim 12, wherein saidat least one additive is a wetting agent.
 14. A process according toclaim 3, wherein the membrane is contacted with a cleansing agentcomprising at least one liquid selected from the group consisting ofwater and alcohols containing from 1 to 6 carbon atoms.
 15. A processaccording to claim 1, wherein the membrane is composed of at least onecellulosic material selected from the group consisting of cellulosenitrate and cellulose acetate.
 16. A process according to claim 1,wherein the membrane exhibits a pore width of from 0.01 to 12 μm.
 17. Aprocess according to claim 16, wherein membrane exhibits a pore width ofgreater than 0.45 μm.
 18. A surface-refined membrane composed ofcellulose, produced by a process comprising the steps of: (a) preparinga feedstock membrane from a membrane casting solution by phase inversionin an evaporation process, and (b) prior to drying the resultingfeedstock membrane, removing impurities from at least one side of thefeedstock membrane from which solvent has been evaporated in theevaporation process of step (a).
 19. A surface-refined membraneaccording to claim 18, wherein the removal of impurities is continueduntil the membrane is free of unevaporated solvent.
 20. Asurface-refined membrane according to claim 18, wherein step (b)comprises bringing the side of the membrane from which impurities are tobe removed, into contact with at least one cleansing agent or cleansingdevice.
 21. A surface-refined membrane according to claim 20, whereinthe impurities are removed by a cleansing fluid.
 22. A surface-refinedmembrane according to claim 21, wherein the cleansing fluid is a gas.23. A surface-refined membrane according to claim 22, wherein theimpurities are removed by a pressurized gas stream.
 24. Asurface-refined membrane according to claim 22, said gas is air.
 25. Asurface-refined membrane according to claim 21, wherein the cleansingfluid is a liquid.
 26. A surface-refined membrane according to claim 25,wherein the impurities are removed by a pressurized liquid stream.
 27. Asurface-refined membrane according to claim 20, wherein the membrane iscontacted with at least one cleansing device selected from the groupconsisting of wipers and suction devices.
 28. A surface-refined membraneaccording to claim 18, wherein, in step (b), the membrane passesthrough: (i) a first rinsing device; (ii) a second rinsing device; and(iii) at least one wiper.
 29. A surface-refined membrane according toclaim 28, wherein membrane in the second rinsing device is contactedwith at least one additive.
 30. A surface-refined membrane according toclaim 29, wherein said at least one additive is a wetting agent.
 31. Asurface-refined membrane according to claim 25, wherein the cleansingagent comprises at least one liquid selected from the group consistingof water and alcohols containing from 1 to 6 carbon atoms.
 32. Asurface-refined membrane according to claim 18, wherein the membrane iscomposed of at least one cellulosic material selected from the groupconsisting of cellulose nitrate and cellulose acetate.
 33. Asurface-refined membrane according to claim 18, wherein the membraneexhibits pore widths from 0.01 to 12 μm.
 34. A surface-refined membraneaccording to claim 33, wherein the membrane exhibits a pore width ofgreater than 0.45 μm.
 35. An apparatus for refining a surface of amembrane composed of cellulose produced by phase inversion in anevaporation process, said apparatus comprising: at least one membranedrawing machine for producing a feedstock membrane by phase inversion inan evaporation process; at least one membrane cleansing device forbringing at least one side of the feedstock membrane from whichimpurities are to be removed, into contact with at least one cleansingagent or cleansing device, and at least one membrane dryer; wherein saidmembrane cleansing device is arranged between the membrane drawingmachine and the membrane dryer.
 36. An apparatus according to claim 35,wherein the membrane cleansing device comprises at least one rinsingdevice for rinsing the side of the feedstock membrane from whichimpurities are to be removed, with a cleansing liquid comprising atleast one cleansing agent.
 37. An apparatus according to claim 36,wherein a first rinsing device is arranged immediately downstream of themembrane drawing machine and a second rinsing device is arrangedimmediately upstream of the membrane dryer.
 38. An apparatus accordingto claim 37, wherein said first rinsing device comprises at least onenozzle for spraying a washing liquid on the side of the membrane fromwhich impurities are to be removed.
 39. An apparatus according to claim37, wherein said first and second rinsing devices each comprise awashing basin for holding a cleansing liquid, and guide rollers fortransporting the membrane through the respective washing basins.
 40. Anapparatus according to claim 39, wherein the washing basin of the secondrinsing device contains a washing liquid, which, in turn, contains atleast one additive.
 41. An apparatus according to claim 40, wherein theadditive is a wetting agent.
 42. An apparatus according to claim 35,wherein the membrane cleansing device comprises at least one wiper orsuction device.
 43. An apparatus according to claim 42, wherein eachwiper comprises a wiper element having a width at least equal to themembrane and arranged on a resilient element so that when the membraneis moved relative to the wiper, the wiper element will sweep over theside of the membrane from which impurities are to be removed.
 44. Anapparatus according to claim 42, wherein the membrane cleansing devicecomprises a wiper roller with wiper elements arranged on an outersurface thereof, said wiper roller rotating in the direction in whichthe membrane is drawn, and at a speed greater than the speed at whichthe membrane is drawn.
 45. An apparatus according to claim 44, whereinmembrane cleansing device further comprises a support roller whichrotates in the direction in which the membrane is drawn and at the speedat which the membrane is drawn, said support roller being arranged toform a nip with the wiper roller through which the membrane is guided.46. An apparatus according to claim 43, wherein each wiper elementcomprises a rubber wiper or a brush.
 47. An apparatus according to claim36, wherein said rinsing device comprises a wash basin containing acleansing liquid and at least one wiping element arranged above the washbasin to wipe cleansing liquid from the membrane emerging from the washbasin.
 48. An apparatus according to claim 43, wherein said wiperelements are provided with nozzles for spraying a rinsing liquid ontothe wiper elements.
 49. A method of diagnostic testing for an analyte ina liquid test sample where the presence or absence of said analyte isindicative of a disease state, said method comprising contacting saidtest sample with a test strip containing a reagent which interacts withsaid analyte in a detectible manner, wherein said test strip comprises amembrane according to claim 18.